Neurotrope X was a large neurotropic virus, able to infect humans. It was visible with a microscope. Symptoms of the infection included fever, coma and, eventually, death. It was employed as a biological weapon by the Cybermen.
In 2070, on Earth's Moon, the crew of the Moonbase was infected with Neurotrope conveyed through the sugar supplies. Patients were put in quarantine. The patients were abducted by the Telosian Cybermen for cyber-conversion. (TV: The Moonbase)
Upon returning to the Nerva Beacon, Fourth Doctor, Sarah Jane Smith and Harry Sullivan came across a group of Cybermen administering a similar virus to the workers of the Beacon via the Cybermats. (AUDIO: Return of the Cybermen)